This invention relates to a pipe lay vessel with a reel for carrying a length of pipe and first drive motor for controlling rotation of the reel, a substantially vertical tower comprising at its upper end a radius controller for deflecting the pipe from a first trajectory extending between the reel and the radius controller to a second substantially vertical trajectory and an adjustable tensioner for carrying the vertical pipe section having drive motors for feeding the pipe along the vertical trajectory. Such a vessel is known from WO-96/35902 in which a pipe is spooled from a storage reel to a bending shoe mounted on a derrick structure that is situated above a moon pool. The vertical pipe section is passed via a 3-point straightening assembly into the water. The known J-lay vessel is particularly suitable for laying pipe in deep water. It is known to suspend the vertical pipe section from a clamping device or tensioner, which can be driven or braked to feed the vertical pipe section towards the seabed. The reel supplying the pipe is also driven or braked in a controlled manner. Variations in pipe speed along the first trajectory between the reel and the radius controller will occur because the pipe coming from the reel will have varying speed, even at constant reel rotation since the pipe is not always evenly spooled on the reel and the diameter of the unspooled pipe on the reel varies during the unspooling process. Also during start-up and stopping and during emergency crash-stop situations speed variations of the pipe will occur along the first trajectory and along the vertical trajectory where the pipe is fed through the tensioner. Speed variations may also be caused by plasticity of the pipe and partial straightening upon unspooling. Because of the varying tension along the first trajectory between the reel and the radius controller sagging of the pipe or overload of structural elements that guide the pipe may occur. Another problem that can occur during laying of pipe is a failure of the tensioner to hold the pipe resulting in an overload of structural elements that guide the pipe, eventually a complete loss of the pipe or in the worst case even loss of human lives. Especially dangerous is the situation where the pipe moves uncontrolled with high speed to the seabed causing unacceptable loads to the supporting structure. Different sizes of pipe can be installed on the seabed by the same vessel. For each size of pipe the tensioner has to provide enough holding force to hold the pipe without damaging the pipe. In current systems changing from one pipe size to another involves a change of the holding shoes of the tensioner. This is a time consuming process with severe consequences if not performed adequately.
It is therefore an object of the present invention to provide a pipe lay system with which tension of the pipe along the first trajectory can be maintained substantially constant regardless of any differences in pipe speed between the tensioner and the reel.
It is a further object of this invention to provide a tensioner system, which can hold several diameters of pipe without the need to change the holding shoes and still provide enough holding force.
Again it is a further object of this invention to provide a system, which precisely aligns the tensioner and holding shoes around the pipe to avoid damage to the pipe and to provide an adequate holding force under all circumstances.
Again it is a further object of the present invention to provide a braking device to hold the pipe when the tensioner has failed to hold the pipe when the pipe moves at low speeds. It is a further object of this invention to provide a braking system, which is capable of braking and holding a fast moving pipe.
Thereto the pipe lay system of the present invention is characterized in that a movable tensioner is provided for lengthening of shortening the first pipe trajectory to maintain a substantially constant tension of the pipe along the first trajectory during unwinding of the reel. The tensioner is provided with a separate braking system to hold the pipe when the tensioner fails. The tensioner of the present invention is provided with a passive aligning system to align the track bodies and holding shoes in the most optimal position for a given pipe diameter. Thus ensuring that always the maximum holding force is generated.
By displacing the tensioner, the first pipe trajectory can be lengthened or shortened to compensate for variations in speed due to non-uniform winding of the pipe and varying diameter of the spooled pipe. As result the tension in the pipe along the first trajectory (the back tension), can be made substantially constant.
In one embodiment of the pipe lay system according to the present invention, a detector is placed near the tensioner for measuring its position relative to a normal working position. A control unit receives position signals from the detector and supplies a control signal to the drive motors of the reel and/or to drive motors of the tensioner for changing the pipe laying speed such that the tensioner is at least substantially returned to its normal position. The tensioner is supported by an adjustable support, which can be adjusted to maintain a certain value of back tension and makes it possible for the tensioner to move. In normal operation the rod of the adjustable support rests on a support mounted on the tower and the tensioner remains stationary. In this way any increase in the downward forces is not transmitted to the pipe section between the reel and the tensioner and cannot influence the back tension. Only when the tension in the pipe section between the reel and the radius controller becomes larger than a pre-set value of the adjustable support, the tensioner will move such that the first trajectory is shortened causing the back tension to remain at a constant value. When this is the case the control unit may either increase the unwind speed of the reel, such that the tensioner is moved back to its equilibrium position on the tower or may decrease the speed at which the pipe is fed along the vertical section. In both cases the tension in the pipe section between the reel and the radius controller will remain at a constant value.
Detecting the position of the tension compensator may be carried out optically of by encoders measuring a hinge angle of the tensioner or, in another embodiment, by measuring the vertical displacement of the tensioner.
By engaging the braking system on the pipe the pipe can be held stationary until the tensioner can hold the pipe again. In one embodiment of the braking device according to the present invention the braking system consists of two (or more) rollers, which engage a moving pipe with such force that the pipe plastically deforms and the outer diameter of the pipe decreases. The deformation of the pipe will cause the moving pipe (which has to provide the energy for the deformation) to stop. An advantage of the roller system is that the pipe can be stopped even when the pipe is moving at high speeds without excessive forces on the braking mechanism or the supporting structure. In another embodiment of the braking device the rollers are replaced with a self-locking braking pads. The braking pads are of such design that the braking force increases with the weight of the length of pipe that needs to be supported but does not exceed a braking force which causes damage to the supporting structure.